This invention relates generally to air intakes for engines, and in particular to an air induction system having an improved mechanism for control of one or more intake doors.
An engine for aircraft propulsion requires intake air that is free from contaminants to provide for efficient combustion and avoid internal damage. The compressor and turbine are designed with small clearances between moving parts which maximize efficiency, but which also increase vulnerability to damage from small foreign particles. Contamination of intake air, even in a small amount, causes premature wear on engine components, increases maintenance costs, and degrades operational performance and reliability. Unfortunately, aircraft are exposed to contaminants when operating at low altitudes where air is frequently contaminated with material from the ground, such as sand and dust. That problem is aggravated for helicopters due to rotor downwash and prolonged low-altitude operation. Systems which remove foreign particles from intake flow have been developed to protect the engine from damage. A contaminant separator, such as a filter, is positioned across the intake of the engine.
Some intake systems have a bypass door which provides an alternate entryway for air. The bypass door is normally sealed to the alternate entryway through the use of a thin, flexible gasket thereby preventing contaminants from leaking into the filtered air stream. During normal operations, the door remains closed. But if the engine requires a quantity of air greater than it receives through the primary intake, such as when the contaminant separator becomes partially or fully clogged, the door is opened to permit continued intake of air and safe operation of the engine. Movement of the door between a closed position and an open position can be controlled through a motorized actuator with internal limit switches that detect the end of travel and turn off the motor, thereby stopping the motion of the actuator ram. Typically, the actuator is directly connected to the door. Activation of the motor moves the door to a desired position, whereupon the motor should automatically shut off.
In order to properly compress the door gasket, the actuator must be stopped at a precise position, often within 0.01 inches. In actual use, the internal limit switch often fails to stop the actuator motor at this precise position. Variations due to ambient temperature change, part tolerances, rigging tolerances, and supply voltage differences can create fluctuations in the position of the actuator at which the motor stops. Consequently, the motor is susceptible to continue running even after the bypass door has closed, causing burnout of the actuator motor and/or excessive force on the door and structure. Alternatively, the limit switch is subject to cut power to the motor before the door is fully closed allowing contaminants to enter the filtered air stream.